In blowmolding processes, it is a common practice to extrude a thermoplastic synthetic resin material through a nozzle head or die and thereby form an elongated parison of a certain wall thickness, the parison being clamped, after extrusion, in a closable blowmold. Gas is introduced under pressure to the interior of the parison, thereby expanding the same to conform to the contours of the cavity of the blowmold. The parison can be formed by feeding the thermoplastic material to the head from an extruder and is produced over a predetermined time period corresponding to the extrusion of the thermoplastic material of the parison through the head or die. The parison is usually tubular and can be engaged by edges of the blowmold, e.g., along the bottom of the parison, when the latter is extruded downwardly so that these edges of the blowmold close off the interior of the parison to enable the blowing thereof.
In the fabrication of large hollow bodies, such as canister and tubs or barrels, which can have volumes of 200 liters and more blowmolding techniques of the types described above are employed although practical application of the technique has been found to give rise to certain disadvantages. More specifically, the parison can be clamped between edges of the blowmold and then must be stretched outwardly to conform to the contours thereof, the clamping action thermally sealing or welding the corresponding edges of the parison together. During the blowing process, the degree of stretching varies along the walls of the parison, from a relatively small degree at the end of the sealed edge to a high degree at the center thereof, for example, and whereever, because of the contours of the production to be made, the parison walls then stretch equally. Consequently, the wall thickness of the product is nonuniform, i.e., the more highly stretched portions are thinner than the less highly stretched portions.
This is especially undesirable in the case of round hollow bodies, such as barrels, where the weld or seam formed by the clamped edges is linear and forms the bottom of the barrel. In this case, the parison assumes the configuration of a flat sack with a longitudinal weld seam at its bottom and the sack must be expanded outwardly (radially) while the bottom is stretched into a circular disk configuration. At the central regions of the weld seak, therefore, the synthetic resin material is stretched so greatly that a minimum wall thickness is formed precisely at the point at which weakening is most undesirable. In the outer corners of the parison there is practically no stretching and hence the original wall thickness of the parison remains practically intact. On the other hand, in the region of the center of the seam or weld, the synthetic resin material must stretch substantially over the entire radius of the barrel outwardly and in all directions, thereby forming the bottom of the barrel. A flow of the thermoplastic material from the regions of greater wall thickness to those of smaller wall thickness does not significantly occur and thus it is possible that the wall thickness of a finished hollow body, such as a barrel, can range between 12 mm at the low stretch regions to about 4 mm in the regions of higher stretch.
As a result, the original thickness of the parison must be sufficient to provide the minimum thickness in the low stretch region for the requisite mechanical stability of the hollow body. This leads to excessive wall thickness elsewhere and a higher than necessary consumption of material. Another disadvantage is that, with wall thicknesses greater than necessary, the cooling time must be increased and hence the production rate of the apparatus is limited.
To overcome these disadvantages, it has been proposed to provide the nozzle head with a core and a surrounding shell and to shift the core with respect to the shell and vice versa so that the flow cross section for the thermoplastic material forming the parison has different thickness at different portions of the length of the parison. Such a system is described in German published application DT-AS No. 15 04 03.3.
This, however, results in only a partial improvement since the variation in the wall thickness must extend over the entire periphery of the parison. This, however, does not eliminate the problem of excessive high degrees of stretch is not to occur or excessive dimensioning of the thickness of the seamed edge. In general, therefore, the problem has not been fully solved by the prior-art arrangement described above.